Sensing of volatile organic compounds (VOCs) is widely required in a number of applications that range from medicine to environmental pollution. For example, in medical diagnosis, air exhaled by patients may be used to indicate lung cancer by the levels of various VOCs. Further, the presence of uncontrolled diabetes can be indicated by a sweet odor that is due to excess acetone vapor in the breath [1]. With regard to environmental issues, such as the global increase in ozone (O3) and other photochemical oxidants, many of these issues are related to VOCs-Nitric Oxide (NOx) levels, as well as levels of other air pollutants. Ozone is a source of hydroxyl (OH) radicals which react rapidly with most air pollutants (NOx, SOx, CO, etc.), leading to the formation of VOCs that are linked to air contaminants [2]. Further, some polycyclic aromatic hydrocarbons (PAHs) are carcinogenic VOCs generated by incomplete combustion of mobile sources, electricity generating power plants, and coal combustion. Also, emissions indirectly linked to VOCs correspond to particulate matter (PM), which coexists in a mixture with sulfates, nitrates and organics to form VOCs [2].
Terrorism is associated with the use of explosives and chemical or biological warfare agents. In the area of homeland security, detection of explosive vapors, including vapors of volatile organic compounds, is desirable in airports, underground transportation, and other susceptible areas. In particular, significant attention is being paid to chemical warfare agents such as organophosphates [3]. As such, responding to a chemical or biological attack requires the ability to rapidly detect the agents, including VOCs that may be present.
In concerns of public safety, detection and quantification of ethanol vapors in breath (breathalyzers) is required for drivers under the influence of alcohol [4,5]. Breath and blood alcohol concentrations are linearly correlated in a partition ratio of 2000 and breath and blood alcohol content are relative to the degree of alcohol intoxication. For instance, levels of intoxication are legally considered to be between 0.5-1.0 g·L−1 in blood, which corresponds to 0.25 to 0.5 mg·L−1 in breath (130-266 ppm v/v) [5]. Sensing ethanol is also widely required in fermentation and distillation, either for process control or for avoiding prolonged alcohol exposure.
Other aspects of detecting VOCs are related to new forms of energy production and involve the detection of methanol in direct methanol fuel cells (DMFC) [6]. Lately, vapor-fed fuel cells are gaining more attention due to some advantages over liquid-fed DMFC. Accordingly, detection of methanol vapor is important for controlling the vapor mixture (methanol+water) flow concentration in order to achieve optimal fuel cell operation.
Electronic noses (e-noses) are electronic devices capable of detecting gas and vapor analytes [4, 7-11]. The e-noses typically use a sensor array to discriminate different analytes in a way that mimics the olfactory system in humans and animals. However, the sensitivity and discrimination among different analytes, especially in complex samples, still requires improvements to compete with techniques like gas chromatography/mass spectrometry (GC/MS). Different transducers used in e-noses include piezoelectric [12-18], surface plasmon resonance (SPR) [19], fluorescence-based [16,20,123], and chemiresistive sensors. In the family of chemiresistors, metal-oxides [7,22,23], carbon-black polymers [8], carbon nanotubes (CNTs) [24], nanorods [23, 25-44], and gold (Au) nanoparticles [24-38, 45-48] have been reported.
Films of Au monolayer-protected clusters (MPCs) have been used for VOC detection due to their ease of fabrication, low power consumption, and the ability to tailor the surface properties to alter the response to different vapors, which can then discriminate between analytes in an array format [49]. For example, a dual-chemiresistor GC based on Au MPC films for detection of low vapor concentrations ranging from 0.1 to 24 ppm has been used [37], and functionalized Au nanoparticles with OH-terminal ligands have been used to improve the affinity and sensitivity towards polar vapors [43]. These devices have detected ethanol at 10-20 ppm, while others using Au nanoparticles with other ligands reported a lowest detection limit (LOD) for methanol of 620 ppm [38]. Another report on vapor sensing with MPCs showed a LOD for toluene of 2.7 ppm that was obtained through current conversion, filtering, and rectification [41].
Accordingly, there remains a need in the art for improved sensors and methods for detecting VOCs. In particular, there is a need for more sensitive and selective sensors and methods for detecting VOCs that are capable of high sensitivity, fast response, reproducibility, and stability.